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abs_lobanov [2016/11/04 12:50]
pamwhitfield
abs_lobanov [2020/07/16 11:29]
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-====== Lobanov Absorption Correction for TOF Neutrons ====== 
- 
-Description:​ This is the absorption correction used in GSAS formatted for TOF neutron diffraction 
- 
-Comment: Lobanov absorption macro with calculated lambda for wavelength dependence of mu (as per GSAS manual). The macro can be edited to function with CW data with '​Lam'​ replacing '​lambda'​ and '​Sin(Th)'​ replacing '​sintheta'​ 
- 
-Contributed by: Pamela Whitfield 
- 
-Lf is the flightpath and two_theta the detector angle. The refinable parameter under each dataset is '​mu'​ 
- 
-POWGEN flightpath is 60 metres with data focussed into a single dataset at 90 degrees. ​ 
- 
-Absorption is angle-dependent. For more conventional TOF instruments with multiple detector banks such as HRPD, two_theta (and maybe Lf) will differ for each bank by using '​local'​ variables as opposed to global '​prm'​ variables, e.g.  
- 
-   ​TOF_XYE.... 
-    local !two_theta 145 
-    local !Lf 25 
-    local mu 0 min 0 
-    Abs_Lobanov 
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-   ​TOF_XYE.... 
-    local !two_theta 90 
-    local !Lf 26 
-    local mu 0 min 0 
-    Abs_Lobanov 
- 
-    macro Abs_Lobanov  ​ 
-    { 
-    prm !Lf 60.183 ​ ' POWGEN 90 degree flightpath 
-    prm !h_bar 6.626176e-34 ' Plank'​s constant 
-    prm !m_n 1.67495e-27 ' neutron mass 
-    prm !td_const = 10^4 h_bar/(m_n Lf); 
-    local !sintheta = Sin(Deg_on_2 two_theta); 
-    local !lambda = Xo 10^4 h_bar/(m_n Lf); 
-    local !k0 = 1.697653; 
-    local !k1 = (25.99978 - (0.01911 sintheta^0.5)Exp(-0.024514 sintheta^2)) + 0.109561 sintheta - 26.0456; 
-    local !k2 = -0.02489 - (0.39499 sintheta^2) + (1.219077 sintheta^3) - (1.31268 sintheta^4) + (0.871081 sintheta^5) - (0.2327 sintheta^6);​ 
-    local !k3 = 0.003045 + (0.018167 sintheta^2) - (0.03305 sintheta^4);​ 
-    local !k4 = 1.433902 + (11.07504 sintheta^2) - (8.77629 sintheta^4) + (10.02088 sintheta^6) - (3.36778 sintheta^8);​ 
-    local !k5 = ((0.013869 - 0.01249 sintheta^2)Exp(3.27094 sintheta^2)) + ((0.337894 + (13.77317 sintheta^2))/​((1 + 11.53544 sintheta^2)^1.555039));​ 
-    local !k6 = (1.933433 / (1 + 23.12967 sintheta^2)^1.686715) - 0.13576 sintheta + 1.163198; 
-    local !k7 = 0.044365 - (0.04259/​((1 + 0.41051 sintheta^2)^148.4202));​ 
-    scale_pks = If( (mu*lambda) < 3, Exp(-k0 (mu*lambda) - k1 (mu*lambda)^2 - k2 (mu*lambda)^3 - k3 (mu*lambda)^4),​ 
- Exp((k4 - k7/(1 + k5 ((mu*lambda)-3))^k6)+k7) 
- );  
-    } 
  

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